She slept soundly through the night.
Meaning: This means that she was in a state of rest with her eyes closed in a manner that was deep and peaceful during the period of darkness between sunset and sunrise.
Choice of Words: Slept is a verb that means to be in a state of rest with your eyes closed. Soundly is an adverb that describes the manner in which the sleeping is done. Night is the period of darkness between sunset and sunrise. When you put them together, "Slept soundly through the night" describes the action of being in a state of rest with your eyes closed in a manner that is deep and peaceful during the period of darkness between sunset and sunrise.
Alternative Expressions
Related Expressions